I don't see the Lions ever moving from Detroit... there's too much history there and despite the fact that they rarely do anything of note they have a very dedicated fanbase. But they need to work on their defense, that is for sure.
It's a tricky game picking how you are going to spread the talent on your squad: Put a lot into offense and the defense has to suffer and vice/versa. The only real solution is to get good contracts with underrated college players or players that are underutilized by other teams. This is a very tricky process and sometimes you get lucky, but most of the time you don't.